Biaxial tensile strained silicon (Si) directly on insulator (SSDOI) is a layer of silicon, typically disposed directly on dielectric layer formed on a semiconductor substrate, in which the silicon atoms are stretched beyond their normal interatomic distance. One way of straining silicon is to grow a layer of Si over a substrate of silicon germanium (SiGe). As the atoms in the Si layer align with the atoms in the SiGe layer, wherein the interatomic distances longer, atomic “links” become stretched, or strained (i.e., strained silicon). An increased distance between stretched or strained atoms reduces the atomic forces that interfere with the movement of electrons through transistors formed in the SSDOI, i.e., increased electron mobility when properly biased. The increased electron mobility results in improved transistor operation, and lower energy consumption thereby. For example, transistor devices fabricated in SSDOI exhibit increased electron velocity through the device upwards of seventy percent (70%). Where electrons traverse device conduction paths by around 70% results in upwards of a thirty-five percent (35%) decrease in the time required to switch.
Strained SSDOI substrates are formed by a combination of chemical vapor deposition (CVD) and layer transfer process. Precursors for the CVD process include silicon-derived silane and dichlorosilane, and germanium-derived germane, germanium tetrachloride and isobutylgermane. U.S. Pat. No. 7,314,790 (“the '790 patent”), commonly-owned and incorporated by reference herein, discloses a method of forming biaxial tensile strained SSDOI substrates. The strained SSDOI substrates include a Si-containing layer with any crystal orientation, where crystal orientation (where <100> is the most typical crystal orientation) and biaxial tensile strain. Biaxial tensile strain is a term used to describe a net stress caused by longitudinal and lateral tensile stresses induced in a Si layer at or during the SSDOI substrate formation.
While the increased electron mobility inherent in biaxial tensile strained SSDOI substrates is ideal for N-doped transistor operation, for example, increased switching speeds in NFET devices fabricated therewith, it is not ideal for P-doped operation, i.e., PFET devices fabricated on SSDOI. That is, biaxial SSDOI is not known to enhance hole mobility. For that matter, localized uniaxial compressive strained silicon, or relaxed SSDOI is known to dramatically enhance hole mobility when P-doped, which is ideal for PFET device operation.
Conventional processes for forming biaxial SSDOI, however, are not known to realize uniaxial compressive strain, or portions of SSDOI substrates with uniaxial compressive strain in order to realize an SSDOI substrate from which both NFET and PFET devices may be formed with enhanced respective electron and hole mobilities, and therefore improved switching speeds within the devices so formed.